Are you actually intending to edit the code in the cloned version of the package? Or are you just trying to have two different versions of A as dependencies for two different versions of Julia?
If the latter, don’t develop A; instead Pkg.add the version that you want to pin for the older Julia version.
Note also that you can always download a new version of A to anywhere you want using any method you want. If you then develop(path/toA) you will use that version of A in that particular environment. develop just means: put an entry into Package.toml that points to a local folder. Whatever code sits there will be used in that environment. That could be your own code or a package that you downloaded (by whatever method) from anywhere else.